The invention relates to a windshield wiper system and a method for operating a windshield wiper system.
Windshield wiper systems for cleaning motor vehicle windshields of this generic type are known. One disadvantage of known wiper systems is that when rain quantities are slight, a fine film of water remains on the window to be wiped, which is due primarily to an overly high wiper speed and which can impair the view for the driver. This is known as streaking and is dependent both on the degree of wetting of the window and on the wiper speed. The higher the wiper speed, the greater the tendency to streaking. Wiper systems with only one windshield wiper are especially critical with regard to streaking, because one wiper fundamentally has to sweep over the window markedly faster, if it is to produce the same cleaning action as two wipers.
Known windshield wiper systems usually have only two different wiper speeds and one intermittent position. In the intermittent position, the wiper speed is typically equivalent to the slower position in the continuous mode. The disadvantage then is that when only slight quantities of water are on the windshield, the slowest wiper speed is still too fast, and the wipers therefore tend to streaking.
Provisions for variable control of windshields wipers are known. German Patent DE 33 14 770 C2, for instance, shows a device for controlling a windshield wiper motor, whose action is based primarily on assessing measurement signals of an optical rain sensor such that when a certain minium rain quantity is measured, the windshield wiper is turned on. The wiper speed is also supposed to be capable of being reduced if only slight rain quantities are detected. Since for safety reasons (hindrance to vision for the driver) a minimum speed of the windshield wipers cannot be undershot, there is a gap between wiping at intermittent intervals and the lowest possible wiper speed in the continuous mode. Thus even in the subject of the invention of the aforementioned patent, the fundamental problem of streaking still exists.
Another known provision resides in continuous regulation of the wiper speed by means of a regulating device that is capable of processing the signals of a rain sensor, among other signals, as an input variable. Thus German Patent DE 197 00 457 C1 shows an apparatus and a method for a windshield wiper system in which a drive motor is regulated in such a way that its speed of revolution and thus the speed of the windshield wipers is variable depending on the input variable. Once again, however, an unambiguous quantitative association of certain detected rain quantities with certain wiper speeds is not provided. Thus once again the fundamental problem of streaking still exists.
In accordance with the present invention, in the windshield wiper system, the wiper is operatable with variable intermittent interval times and at variable wiper speeds as a function of a rain quantity, and precisely one intermittent interval time are associated with each measured rain quantity.
The windshield wiper system of the invention offers the advantage that streaking from an overly high wiper speed can be reliably avoided. This can be done by controlling the wiper motor by detecting a rain quantity, preferably with an optical rain sensor. An unambiguous association of each measured rain quantity with a certain wiper speed is preferred; at low rain quantities, the wiper is first operated in an intermittent mode, with variable intermittent interval times, and as the rain becomes stronger it is operated in a continuous mode with a variable wiper speed. In the intermittent mode the wiper operates with a lower speed than in the continuous mode. Controlling the direct-current motor that is typically used can be accomplished by either voltage regulation or by pulse width modulation (PWM), for instance.
By lowering the wiper speed in the intermittent mode to a relatively low value, streaking can be averted. Instead of changing immediately from an intermittent interval time of five seconds, for instance, to the continuous mode when the rain is relatively strong, it is initially advantageous to reduce the intermittent interval times, while the wiper speed remains constantly low. Reducing the intermittent interval time to a value of zero then corresponds to a continuous mode. It is only in the continuous mode that the wiper speed is increased accordingly as the rain quantity increases. The lowest wiper speed preferably has a minimum value, at which a driver still does not perceive the motion of the windshield wiper across his view as a hindrance to his vision. The adaptation to the different measured degrees of wetting of the window can be done either by continuously changing the speed of the drive motor, or in discrete stages.
In a preferred further feature of the invention, it is provided that the speed regulation of the at least one wiper has a fuzzy logic algorithm; as a result, given the fluctuations that necessarily occur in the output signals of an optical rain sensor, the wiper speed or the intermittent interval times can be reliably prevented from being subject to major transient fluctuations.